The potential for primary energy production through the capture and conversion of incident solar energy is immense, principally because of the magnitude of the available resource; the solar flux intercepted by the Earth averages about 1.3 kW/m2. After accounting for atmospheric absorption, the total solar flux falling on the Earth exceeds the worlds total electric power demand by a factor of 50,000. That more of the worlds energy demand is not satisfied by solar conversion is due to the diffuse nature and time variability of the solar flux, and to the immaturity of the technologies required to overcome these limitations.
If appropriate systems can be developed both for collecting and for storing solar energy, it would be possible to satisfy the entire world demand for electricity by covering 22,000 square kilometers of desert with solar collectors. Currently, no more than a few square kilometers of solar collectors are in operation in the world. If the technology of solar power can be made economically competitive with alternate power technologies, the potential for rapid growth in solar is very large.
Two fundamentally different technologies are available for converting sunlight to electricity: photovoltaic and thermodynamic. Photovoltaic technology is familiar to most people, even if only in the form of solar-powered calculators. Photovoltaics convert the incident light to electricity through a quantum mechanical process, and depend on the manufacture of precisely defined surface structures in semiconductors using processes developed extensively over recent decades for the computer and electronics industry. Although significant progress has been made in improving the efficiency and reducing the cost of solar cells, they are still far too expensive to compete with traditional power sources, except in niche applications.
Thermodynamic solar power systems work by using concentrated sunlight to heat a working fluid that is then expanded through a turbine (or other thermodynamic engine) to drive a generator. Because the sunlight is concentrated, these systems are referred to as Concentrating Solar Power (CSP). Except for using solar power as the heat source, the process is analogous to fossil-fueled or nuclear power stations, which also produce electricity by heating a working fluid and extracting power through a thermodynamic engine. Other than the source of the heat, the two systems are functionally identical. In each case, the heat is used to heat a working fluid that is then expanded through a thermodynamic engine (most often a Rankine-cycle engine) attached to a generator that produces electricity. Waste heat is rejected to the environment. There is an extensive installed base of both industrial capacity and technical know-how in thermodynamic power conversion. Solar thermodynamic power systems can take advantage of this installed industrial base if they can be designed to provide heat with characteristics that match the characteristics of the heat produced by fossil-fueled or nuclear power plants. The two key characteristics of process heat are the working temperature and the rate of heat production. Current solar thermodynamic power systems generally fail to match either of these characteristics.
With respect to working temperature, most fossil- or nuclear-fueled systems using steam turbines operate with hot-side temperatures near 600 C. This temperature has become the default for most thermodynamic power systems through many decades of engineering experience and involves compromises among efficiency and cost, the material properties of system components, and the thermodynamic properties of the working fluid (most often water). Because of limitations inherent in current technologies for solar concentrators, most solar-thermodynamic systems operate at a working temperature of less than 375 C. The lower temperature makes them incompatible with the available technology for thermodynamic power conversion operating at temperatures near 600 C.
A solar receiver operating at 600 C will lose energy through infrared radiation at a rate of about 32 kW/m2. Because sunlight falling on the earth's surface carries at most about 1.1 kW/m2, high working temperatures are possible only if the sunlight is concentrated using a reflecting or refracting collector and focused on a receiver with a smaller area. If the infrared losses are to be limited to no more than 10% of the incident solar radiation, the concentration ratio (the ratio between collector area and receiver area; abbreviated CR) must be (at peak solar flux) at least 340. If the effective solar flux is reduced (through atmospheric absorption, or when the sun is not in a direction perpendicular to the aperture of the collector), the required concentration ratio is much larger.
Because the sun is not stationary in the sky, solar concentrators require tracking systems to keep them pointed at the sun, and systems with higher concentration ratios require more precise tracking systems. Solar tracking systems come in two distinct categories. In dual-axis systems, the sunlight is focused to a small spot, and the tracking occurs about two axes to keep that spot focused on the receiver. In single-axis systems, sunlight is concentrated along a linear focus, and the system tracks only the component of solar motion around an axis parallel to that focal line. The component of solar motion around a perpendicular axis is not tracked. Instead, the solar motion in this direction results in the solar image being displaced along the receiver axis. However, since the linear receiver is highly elongated, the shift in the image location does not move the solar image off the receiver (except at the ends), and does not significantly decrease the total energy absorption. The four most common types of solar concentrators include two dual-axis systems and two linear systems.
The dual-axis paraboloidal dish collector typically includes a large paraboloidal reflector built either as a single unit or as an array of individual mirrors on a single frame, concentrating sunlight at the focus of the dish. Typical dish systems will have a CR of 2000 or more. The main drawbacks of dish systems are that they require high-precision tracking systems operating with complete freedom of movement in two axes; both the dish and the receiver have to move, making the tracked mass very large; and the receiver is isolated at the end of a long arm, making it difficult to collect the heat at a large-scale central power conversion facility, requiring therefore a power conversion unit on each dish. The dual-axis solar power tower consists of a central receiving unit typically located on a tower, surrounded by a field of flat mirrors, each on a pedestal with a two-axis tracking system. While this design overcomes some of the deficiencies of the paraboloidal dish systems, particularly by having a stationary central receiver, there are still drawbacks due to the requirement for high-precision two-axis tracking systems on each of the mirrors. The requirements are made further complex because no two of the mirrors are in the same location relative to the receiver, so each mirror must track the sun in a slightly different way. In addition, because of issues with shading (where sunlight falling on a mirror can be reduced because it is first intercepted by a nearby mirror) and blocking (where sunlight reflected by one mirror falls on the back of a nearby mirror rather than on the receiver), the mirror array cannot completely cover the surface around the tower, limiting the fraction of sunlight incident on the ground that can be collected at the receiver (the ground utility factor). Dish collectors do not have issues with blocking, but shading restrictions, as well as restrictions to prevent physical contact between adjacent dishes, also limit their ground utility factor.
Solar concentrators with linear receivers tend to be simpler and less expensive than dual-axis systems, but generally cannot operate efficiently at temperatures compatible with conventional thermodynamic power systems. The single-axis parabolic trough collector includes a linear parabolic trough-shaped reflector and a linear receiver at the focus of the reflector. The system, including both reflector and receiver, rotates about an axis parallel to the receiver to track one component of the solar motion (typically the east-west component). Parabolic trough collectors are relatively easy to build, are modular, and lend themselves well to scaling up to very high power levels. As such, the majority of worldwide installed CSP systems are of this basic type. Because of the finite angular diameter of the sun, the maximum CR theoretically possible with single-axis concentration is about 215. In practical parabolic trough systems, the CR rarely exceeds 60, which is clearly not high enough to achieve a working temperature of 600 C. In addition, adjacent troughs are typically spaced at least four trough widths apart to avoid shading, so the ground utility factor is not more than 25%. The single axis linear Fresnel collector includes a stationary linear receiver and an array of linear reflectors that each individually tracks the sun by rotating about an axis parallel to the receiver axis in order to keep the image of the sun focused on the linear receiver. Compared to parabolic trough systems, which have moving receivers, the linear Fresnel systems are simpler because the receiver is stationary and only the reflectors need to move. In addition, because the location of the reflector is fixed relative to the fixed receiver, the required angular rotation of the Fresnel reflectors is only half that of the parabolic trough reflectors. The lower moving mass, the smaller angular displacement, and the reduced exposure to wind loads means that the tracking system for the linear Fresnel systems is significantly simpler and less expensive than for the parabolic trough systems. While linear Fresnel systems typically have ground utility factors of 90% or greater, they do have issues with shading and blocking. The overall efficiency of a typical linear Fresnel system is less than that of a parabolic trough system. No known practical way is available to build a linear-receiver solar concentrator with a working temperature of 600 C or higher.
With respect to the rate of heat production, the key difference between conventional (fossil or nuclear) thermodynamic power systems and CSP thermodynamic power systems is that conventional systems can run continuously, and some are designed specifically to stop and start on demand for satisfying peak loads, while solar concentrators provide heat only when the sun is shining.
While means are available for storing thermal energy to allow conversion to electricity on demand, these means are relatively inefficient and expensive, and can typically store only enough heat for a few hours of electricity production. Various methods have been proposed in the past several decades for storing thermal energy for solar power applications, including storage as sensible heat (heat associated with a temperature change in an otherwise unchanging material), latent heat (heat associated with a phase change), and in reversible thermochemical reactions. In principle, almost any material can provide a basis for sensible heat storage. In practice, the material must be stable over the entire temperature range of interest, and it would preferably have a high specific heat on a mass or volume basis, and be readily available, safe, inexpensive, and environmentally benign. If the storage material is liquid, it requires an appropriate storage tank (including pressure containment if the liquid has a high vapor pressure). If the storage material is solid, some means must be developed for transferring the heat into and out of the storage material. In latent heat storage systems, heat added to the system is absorbed (at a constant temperature) as heat of fusion when the storage medium melts. Heat is given up when the storage medium solidifies. These systems require both a tank to contain the storage medium when it is in the liquid phase, and a method for moving heat in and out of the system that is compatible with both solid and liquid phases of the storage medium. These systems can store heat only at the temperature of the phase change, and a storage medium is needed with a phase change in the right temperature range. For single-axis linear collectors such as the parabolic trough and the linear Fresnel systems, which normally use a liquid-phase working fluid circulating through the receiver to collect the solar energy, the most commonly proposed storage method is simply a tank to contain a large volume of the hot working fluid. While conceptually simple, this method is constrained by the cost of the working fluid. The volume of fluid stored in the tank can be reduced by including a large mass of solid particles in a packed-bed thermal storage system. In this configuration, only the void area between the particles is filled with the working fluid. However, since practical packed-beds have void fractions in excess of 30%, there is still a requirement for a large volume of working fluid to achieve substantial thermal storage. No known practical way is available to provide thermal storage for solar concentrators sufficient to provide several days of electric power production.